Okanagrion is an extinct odonate genus in the damselfly-like family Dysagrionidae. The genus was first described in 2021 with a series of eight species included from early Eocene Okanagan Highlands sites in western North America. The genus is known from the Late Ypresian sediments exposed in northeast central Washington at Republic where five species are present, and from the coeval McAbee Fossil Beds near Cache Creek in Central British Columbia, where four species are present. The species richness is attributed to high latitude high alpha diversity resulting from climatic equitability during the Early Eocene in combination with resultant beta diversity between sites due to impassible topographical barriers.
Distribution
Okanagrion fossils have been found at two sites belonging to the Eocene Okanagan Highlands of Washington and British Columbia. Of the eight described species, O. angustum, O. beardi, and O. lochmum are only found at the Tranquille Formation's McAbee Fossil Beds west of Cache Creek in central British Columbia. The lake sediments at McAbee were first radiometrically dated using the K-Ar method in the 1960s based on ash samples exposed in the lake bed. These samples yielded an age of ~; however, dating published in 2005 provided a 40Ar-39Ar radiometric date placing the McAbee site at .
Four additional species are known exclusively from fossils of the Klondike Mountain Formation exposed at Republic in Ferry County, northeast Central Washington. Two species, Okanagrion dorrellae and O. liquetoalatum are known only from the "Boot Hill" site UWBM B4131, while O. threadgillae is only known from the "Corner lot" site A0307 and one species, O. worleyae is reported from both the "Boot hill" and "Corner lot" sites. Tuffs of the Klondike Mountain Formation had been dated to , the youngest of the Okanagan Highlands sites, though a revised oldest age of was given based on isotopic data published in 2021.
The widest distribution is that of Okanagrion hobani, for which multiple fossils have been recovered from both Republic sites and at McAbee, the only Okanagrion species to be found in multiple formations.
History and classification
The first brief report of an Okanagan Highlands fossil odonate was by Standley Lewis in a 1992 Washington Geology article where he illustrated two fossils tentatively identified as (?)Zacallites from Republic. Four years later a list of insects from Republic was published by Wesley Wehr and Lisa Barksdale in a 1996 Washington Geology article. They expanded the tentative families to include Euphaeidae, Megapodagrionidae, Platycnemididae or Megapodagrionidae, and Lestidae but did not designate the fossil specimens each identification was based on. The odonate fossil material was studied subsequently by S. Bruce Archibald and Robert Cannings with a series of papers being published between 2019 and 2021 on the dragonflies and damselflies of the Okanagan Highlands. The new genus Okanagrion was named by Archibald and Cannings (2021) with a series of eight named species and a group of unplaced fossils which were not placeable to species. They coined the genus name Okanagrion as a combination of a toponym honoring the Okanagan Highlands plus the suffix "-agrion", from the Greek "ἄγριος", commonly used in damselfly genus names.
Based on shared characters of the wings, such as a missing crossvein O and the CuA–A space being expanded, Archibald and Cannings placed the new genus into the extinct family Dysagrionidae. Separation of the two dysagrionid subfamilies is based on the basal origin point of wing vein RP3-4, in the Petrolestinae species this is positioned midway between the arculus and subnodus veins, while in Dysagrioninae species the placement is around 2/3 of the distance towards the subnodus from the arculus. additionally the IR2 vein in petrolestines branches from very near or on the RP3-4, but forks at or on the subnodus in Dysagrionines. With Okanagrion the RP3-4 originates near the 2/3 point towards the subnodus, while the IR2 forks from or directly basal to the subnodus.
The subordinal placement of Dysagrionidae is controversial. Until 2021 the family had been treated as likely belonging to the living odonate suborder Zygoptera (damselflies), however the known head morphology of the included species has led to occasional placement questions. The Cretaceous dysagrionid genus Congqingia has had affiliation with the suborder Anisozygoptera and Dysagrionidae has been noted to be Zygoptera(?) on occasion. Archibald, Cannings, and Robert Erickson evaluated the known head morphology of the fossils and concluded that they did not belong to any of the three defined odonate suborders, but instead were part of a new suborder they named Cephalozygoptera. The suborder was based on the zygopteran body plan, but considered distinct due to the eye to head capsule structure and distances. Unlike zygopterans where the head is 3-5 times as wide as long, in cephalozygopterans the heads are only around double as wide as long making them narrower than in any zygopterans. The distance between the eyes in cephalozygopterans, at about one eye width, was also suggested to be different from known zygopterans where the width is typically at more than doubled. The erection of Cephalozygoptera was criticized by Andre Nel and Darren Zheng (2021) who argued that the noted differences were due in part to deformation of the head and eye areas during deposition and fossilization combined with characters that are present in a larger group of odonates than Archibald, Cannings, and Erickson reported. Based on their review and critique of the justifications for cephalozygoptera, Nel and Zhang considered the name and grouping unwarranted and proposed it to be treated as a synonym of Zygoptera. Archibald and Cannings (2021) responded to the arguments, maintaining that Cephalozygoptera as valid. In subsequent papers both suborders have been used, depending on if the author group includes Archibald or Nel.
Description Okanagrion species are united by a series of features that are not seen in total in any other Dysagrioninae taxon. Okanagrion species wings have a pterostigma which is between 3 and 3.5 times wider than tall, with a few exceptions where the width is 4 times wider. The genus is missing the oblique brace vein, which is present or unreported for the majority of dysagrionine species and there are no accessory crossveins immediately basal or apical to the CuP. The costal space apical of the pterostigma is between three and four cells wide. The petiole section of the wings ends basally to the CuA and CuP veins.
Between the species, coloration and patterning of the wings are widely varied, most species having broad areas of either light to clear windows on a darkened background, or conversely smaller areas of darkened membrane on light to clear background wing. At least one species O. liquetoalatum is noted to have a fully hyaline wing. In all species, the fore and hindwings are very similar, so differentiation is difficult. Based on the associated wings of the O. threadgillae holotype, Archibald and Cannings treat the assumed hindwings as having a basal hind margin as being smoothly curved from base to level with the quadrangle cell, while the front margin is nearly straight between the nodus and the pterostigma. In the assumed forewings the basal hind margin is mildly convex and widened when reaching the level of the quadrangle cell base, while the front margin between nodus and pterostigma is slightly curved.
Okanagrion angustum Okanagrion angustum is one of the four species found at the McAbee Fossil Beds, being known at the time of description from only the holotype fossil, Royal British Columbia Museum specimen "RBCM.EH2017.050.11.2491" found by John Leahy. Archibald and Cannings coined the specific epithet from the Latin angustum meaning "narrow" in reference to the overall wing shape estimate.
The specimen is a partial wing indeterminate regarding sex and being forewing or hindwing, as it is missing the basal third of the wing and areas of the apical two-thirds. The wing is overall narrower at the point of the nodus, wide, than most other Okanagrion specie. Only two species have a narrower nodus point, O. worleyae and O. dorrellae at and respectively. The O. angustum wing has an IR2 vein which forked from the RP1-2 vein origin, a fork point only shared with O. beardi, while the other species have fork points between the RP1-2 and RP3-4 veins. O. angustum also differs from O. beardi and O. dorrellae in having a longer length of the Ax1 vein to the nodus.
While the coloration is similar to O. hobani, the darkened apical area is shorter and the dark mid-wing stripe does not seem to extend fully to the hind margin, but only covers a u-shaped region extending from the front margin.
Okanagrion beardi The McAbee species Okanagrion beardi is known from two specimens, the holotype, "F-791", in the Thompson Rivers University collection, and the paratype, "RBCM P1546" of the Royal British Columbia Museum collections. Both fossils were recovered from the Hoodoo Face beds, with the collector and collection date of "F-791" unknown, while "RBCM P1546" was collected in June 1998 by an 8th or 9th grade student and donated to the RBCM by Graham Beard. Archibald and Cannings chose the species name "beardi" in recognition of Beard for his long dedication and energy towards British Columbian paleontology.
Specimen "F-791" has the better preserved wing venation with one full forewing and portions of at least two other wings, possibly all three present. In addition there are fragments of the abdomen, part of the thorax, and possibly part of one eye preserved as a disarticulated grouping around the wings. The forewing is wide by long from the arculus to wing tip, and is only missing a few small areas of apical area and near the nodus. The distance from the nodus to pterostigma is , while the distance from the arculus to pterostigma is . The overall wing outline on the apical side of the nodus is oval with symmetry centered on the lateral midline. The coloration is similar to O. hobani with the basal area of the wings hyaline, the apical areas darkened, with a light colored to hyaline fascia. While the basal hyaline area also encompasses the basal 2/5 of the wing, O. beardi differs from O. hobani in the size of the fascia which is only 1/2 as wide in O. beardi. Additionally in some specimens of O. hobani the fascia extends across the will wing width, while it doesn't fully cross the wing in O. beardi.
For "RBCM P1546" the wings are mostly absent, with only one well preserved forewing, and a basal section of one hind wing present. However, the head, thorax and the base of the abdomen are all present, along with scraps of the legs. The head is fairly well preserved, though labrum and clypeus are both indistinct and lacking detail. A width across the head at the widest point is reported and the eyes are given dimensions. The distance between the eyes is . The thorax has an overall dark coloration, with a possible light stripe running over the mesopleural area on the mesothorax. The leg sections present are too badly damaged and disarticulated to tell any detail. The abdomen is only present as sections one and two. While the underside of the segment may have remnants of the secondary male genitalia, there isn't enough detail left to confirm the sex of the specimen.
Okanagrion dorrellae
Okanagrion dorrellae is known from the lone holotype fossil collected from the Klondike Mountain Formation and housed in the Stonerose Interpretive Center collections as "SR 93-11-02". The specimen was collected and donated by Laurie Dorrell on October 31, 1992, and as such Archibald and Cannings chose to name the species dorrellae in recognition.